Signal processors, such as image signal processors, typically employ an analog front end (AFE) to convert analog sensor information into digital code, which may then be manipulated by a digital signal processor. In some conventional AFEs, the sensor information is sampled by a correlated double sampler (CDS). To account for differences in channel gains, the output of the CDS is generally passed to a programmable gain amplifier (PGA) that can provide independent signal gain or attenuation to each channel. The resulting signal may then be provided to an analog-to-digital converter (ADC) for conversion into a digital signal.
The most common sensors for image signal processors produce signals for three colors: red, green and blue. Therefore, the AFE for such a system has to be able to process three colors simultaneously. To accomplish this in a conventional AFE, three completely separate analog channels, each with a CDS, a PGA and an ADC, are implemented in the AFE. If there are additional sensor outputs (e.g., even and odd for each color), the total number of CDS, PGA and ADC blocks increases proportionally. Each CDS block and each PGA block has an operational amplifier that consumes power even when the operational amplifier is not being used (e.g., when the block is sampling input signals). Therefore, as the number of these blocks increases, both the power being used for amplification and the power being wasted when no signals are being amplified also increases. In addition, as the number of ADC blocks increases, the amount of power consumed and the area requirements for the AFE increase as well.
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